Object model fitting using manifold constraints

ABSTRACT

Aspects of the present invention include systems and methods for forming generative models, for utilizing those models, or both. In embodiments, an object model fitting system can be developed comprising a 3D active appearance model (AAM) model. The 3D AAM comprises an appearance model comprising a set of subcomponent appearance models that is constrained by a 3D shape model. In embodiments, the 3D AAM may be generated using a balanced set of training images. The object model fitting system may further comprise one or more manifold constraints, one or more weighting factors, or both. Applications of the present invention include, but are not limited to, modeling and/or fitting face images, although the teachings of the present invention can be applied to modeling/fitting other objects.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is related to the following concurrently filed,co-pending, and commonly assigned applications: U.S. application Ser.No. 12/392,808, filed Feb. 25, 2009, entitled “Model-Based Object ImageProcessing,” listing Jing Xiao and Derek Shiell as inventors; U.S.application Ser. No. 12/392,820, filed Feb. 25, 2009, entitled“Iterative Data Reweighting for Balanced Model Learning,” listing JingXiao and Derek Shiell as inventors; U.S. application Ser. No.12/392,828, filed Feb. 25, 2009, entitled “Combining Subcomponent Modelsfor Object Image Modeling,” listing Jing Xiao and Derek Shiell asinventors; and U.S. application Ser. No. 12/392,840, filed Feb. 25,2009, entitled “Subdivision Weighting for Robust Object Model Fitting,”listing Jing Xiao and Derek Shiell as inventors. Each of theabove-referenced applications is incorporated herein by reference in itsentirety.

BACKGROUND

A. Technical Field

The present invention pertains generally to image processing, andrelates more particularly to model-based image processing.

B. Background of the Invention

Emerging communications trends point to streaming video as a new form ofcontent delivery. These systems are implemented over wired systems, suchas cable and wireless networks, such as wireless Internet and cellphones. These communications systems require sophisticated methods ofcompression and error-resilience encoding to enable communicationsacross bandwidth-limited and noisy delivery channels. Additionally, thetransmitted video data must be of high enough quality to ensure asatisfactory end-user experience.

Traditionally, video compression makes use of temporal and spatialcoherence to reduce the information required to represent an image. Inmany communications systems, the communication channel is characterizedby a probabilistic model, which describes the capacity or fidelity ofthe channel.

In many communication scenarios, such as online conferencing, human faceimages comprise a large percentage of the visual data. A goodunderstanding of human face images is important in order to achieve goodperformance on such applications such as, for example, enhancing thedisplay quality or recovering image errors due to missing data orcompression effects in the video streams.

SUMMARY OF THE INVENTION

Aspects of the present invention include systems and methods forgenerating models and/or for utilizing those models. Applications of thepresent invention include, but are not limited to, its use inembodiments of systems and methods for modeling face images. Embodimentsof the present invention employ a 3D active appearance model (AAM) facemodel comprising an appearance model formed from the combination of aset of appearance models of facial subcomponents and a global 3D shapemodel that constrains the appearance model. Although embodiments of thepresent invention are described with respect to modeling faces, it shallbe understood that the present invention may be applied to modelingand/or fitting other items beside faces.

In embodiments, a three-dimensional (3D) active appearance model (AAM)face model that is based on a combination of a set of appearance modelsof facial subcomponents and a 3D shape model may be generated using atraining data set of face images. In embodiments, an object modelfitting system comprising a 3D AAM may be generated.

In embodiments, a set of features or control points for an image may beidentified and assigned labels. In embodiments, the labeled set ofcontrol points may provide the basis for subdividing a face image into amesh comprising a set of triangular regions. In embodiments, a set ofmeshes may be generated based on the training data set of face images.

In embodiments, a set of 2D shape models may be generated based on theset of meshes. In embodiments, a set of reconstructed 3D shape modes maybe generated for each of the object images in the training data set byapplying factorization, such as a non-rigid structure from motion(NRSFM) method, to the set of 2D shape models. In embodiments, at leastone symmetry constraint, such as, by way of example, a symmetryconstraint associated with 3D depth, may be applied when generating the3D shapes.

In embodiments, a face appearance model comprising a set of facesubcomponent appearance models may be derived from the training data setof face images. In embodiments, the triangular regions of 2D face imagesmay be grouped into larger regions representing facial subcomponents. Anappearance model of each facial subcomponent may be derived from thetraining data set of face images by extracting a set of linear texturebases capable of representing each subcomponent in the training data setof face images. In embodiments, the orthogonal texture bases areextracted using Principal Component Analysis (PCA). In embodiments, theset of subcomponent models may also include comprise an appearance modelof the global face image.

In embodiments, a comprehensive 3D AAM face model comprises a set ofsubcomponent image appearance models that are constrained by a global 3Dshape model.

In embodiments, shape and/or appearance models may be improved bycorrecting training data set imbalances. In embodiments, an unbalancedtraining data set of face images may be balanced based on identifyingface images associated with fitting errors that are greater than athreshold. In embodiments, an unbalanced training data set may bebalanced either by adding more of the types of face images havinggreater fitting errors (and increasing the size of the data set), or byweighting the face images having greater fitting errors (and keeping thesize of the data set constant).

In embodiments, shape and/or texture manifolds are generated based onshape and/or texture parameter vectors generated by fitting the objectimages in the training data set to generated shape and/or appearancemodels. In embodiments, an object model fitting system comprising a 3DAAM and a set of manifold constraints based on one or more manifolds maybe generated.

In embodiments, a reconstructed object image may be formed by fitting aninput object image to an object model using an object model fittingsystem comprising a 3D AAM comprising a 3D shape model and an appearancemodel comprising a set of subcomponent appearance models by iterativelyperforming steps that may comprise forming a projected input objectimage by projecting a current 3D shape and pose estimate onto the inputobject image and forming a current reconstructed object appearanceestimate of the projected input object image warped to a canonical view.A fitting error representing a difference between the currentreconstructed object appearance estimate and a current object appearancemodel estimate is determined. Responsive to a stop condition not beingmet, the fitting error is used to form an updated current 3D shape andpose estimate by updating 3D shape parameter values and pose parametervalues and to form an updated current object appearance model estimateby updating appearance parameters. Responsive to a stop condition beingsatisfied, the current object appearance model estimate is designed asthe reconstructed object image. In embodiments, the input object imageis a face image and the 3D AAM is a face model. In embodiments, fittingthe image to the 3D AAM may be performed by an inverse compositionalmethod.

In embodiments, the object model fitting system comprises one or moreweighting factors, and the fitting can be made more robust by includingweighting factors associated with regions of pixels, which may be onepixel or groups of pixels. Robust fitting may comprise updating ofweighting values associated with parameters. In embodiments, robustfitting based on registration error may be implemented by applying aweighting matrix to features of pixels, regions of pixels, and/orsubcomponent regions. In embodiments, pose-based pixel weighting may beapplied so that higher weights are given to those portions of the imagethat most directly facing the camera and/or to give lower weights topixels to pixels with oblique camera angles.

In embodiments, the object model fitting system may comprise a 3D AAMand manifold constraints that may be applied during fitting. Inembodiments, applying a shape and/or appearance manifold may comprisedetermining a parameter value distance to a cluster center in a shapeand/or appearance manifold.

In embodiments, a system for forming a reconstructed object image froman input object image may comprise an object model fitter that receivesthe input object image and forms the reconstructed object image byfitting the input object image to a 3D AAM using an object model fittingsystem. In embodiments, the 3D AAM comprises a 3D shape model and anobject appearance model comprising a set of object subcomponentappearance models. In embodiments, the input object image is a faceimage and the 3D AAM is a face model. In embodiments, the object modelfitter comprises a reconstructed object estimator that receives theinput object image and the 3D AAM and generates a reconstructed objectestimate, and that receives updates to the 3D shape parameter values,pose parameter values, and texture parameter values associated with acurrent reconstructed object appearance estimate and a current objectappearance model estimate. In embodiments, the object model fitter alsoincludes a reconstructed object updater that receives the currentreconstructed object appearance estimate and the current objectappearance model estimate, and that calculates updates to the 3D shapeparameter values, the pose parameter values, and the texture parametervalues. In embodiments, the object model fitter includes robustweighting as part of the fitting process. In embodiments, the objectmodel includes one or more manifold constraints, which are used toconstrain updates to one or more parameters.

In embodiments, the system may further comprise an object model builderthat forms the object model fitting system comprising the 3D AAM basedon the set of training object images. In embodiments, the object modelbuilder comprises a 3D shape reconstructor and modeler that receives 2Dshapes of the set of training object images and forms a 3D shape modelbased on the 2D shapes, and a subcomponent appearance modeler that formsa set of object subcomponent appearance models based upon the set oftraining object images. In embodiments, the object model builder mayfurther comprise a model balancer that balances a training data set ofobject images based on identifying object images associated with fittingerrors that are greater than a threshold. In embodiments, the objectmodel builder may further comprise a manifold builder that generatesshape and/or texture manifolds based on shape and/or texture parametervectors generated by fitting the object images in the training data setto generated shape and/or appearance models.

Some features and advantages of the invention have been generallydescribed in this summary section; however, additional features,advantages, and embodiments are presented herein or will be apparent toone of ordinary skill in the art in view of the drawings, specification,and claims hereof. Accordingly, it should be understood that the scopeof the invention shall not be limited by the particular embodimentsdisclosed in this summary section.

BRIEF DESCRIPTION OF THE DRAWINGS

Reference will be made to embodiments of the invention, examples ofwhich may be illustrated in the accompanying figures. These figures areintended to be illustrative, not limiting. Although the invention isgenerally described in the context of these embodiments, it should beunderstood that it is not intended to limit the scope of the inventionto these particular embodiments.

FIG. 1 illustrates exemplary video communication systems according tovarious embodiments of the invention.

FIG. 2 depicts a block diagram of an object modeling system according tovarious embodiments of the invention.

FIG. 3 depicts a block diagram of a first embodiment of an object modelbuilder according to various embodiments of the invention.

FIG. 4 depicts an exemplary illustration of using Delaunay triangulationto generate a mesh of a face image according to various embodiments ofthe invention.

FIG. 5 illustrates generating 3D shape modes from 2D shapes according tovarious embodiments of the invention.

FIG. 6 illustrates views of a 3D shape model with no symmetry constraintand views of a 3D shape model with a symmetry constraint according tovarious embodiments of the invention.

FIG. 7 depicts a block diagram of a second embodiment of an object modelbuilder according to various embodiments of the invention.

FIG. 8 depicts a block diagram of a third embodiment of an object modelbuilder according to various embodiments of the invention.

FIG. 9 illustrates exemplary unconstrained and manifold constraint faceshapes according to various embodiments of the invention.

FIG. 10 depicts a block diagram of a fourth embodiment of an objectmodel builder according to various embodiments of the invention.

FIG. 11 depicts a block diagram of an object model fitter according tovarious embodiments of the invention.

FIG. 12 depicts an exemplary model fitting process according to variousembodiments of the invention.

FIG. 13 illustrates an exemplary computation of pose-based weighting ofpixels within subdivisions of a face image according to variousembodiments of the invention.

FIG. 14 illustrates an exemplary weighting of pixels based onregistration error for a face image with an occlusion according tovarious embodiments of the invention.

FIG. 15 depicts a first embodiment of a method for generating an objectmodel according to various embodiments of the invention.

FIG. 16 depicts a method for extracting features from an image accordingto various embodiments of the invention.

FIG. 17 depicts a method for generating a 3D shape model from a set ofobject images according to various embodiments of the invention.

FIG. 18 illustrates an exemplary subdivided face image according tovarious embodiments of the invention.

FIG. 19 depicts a method for generating a balanced object modelaccording to various embodiments of the invention.

FIG. 20 depicts a method for generating an object model fitting systemcomprising a manifold constraint according to various embodiments of theinvention.

FIG. 21 depicts a method for building shape and texture manifoldsaccording to various embodiments of the invention.

FIG. 22 depicts a method for generating an object model fitting systemcomprising a balanced model and manifold constraints according tovarious embodiments of the invention.

FIG. 23 depicts a method for using an object model fitting system to fitan object model to an input object image according to variousembodiments of the invention.

FIG. 24 depicts a method for an inverse compositional fitting of anobject model to an input object image according to various embodimentsof the invention.

FIG. 25 depicts a method for applying a manifold constraint according tovarious embodiments of the invention.

FIG. 26 depicts a block diagram of a computing system according tovarious embodiments of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following description, for purpose of explanation, specificdetails are set forth in order to provide an understanding of theinvention. It will be apparent, however, to one skilled in the art thatthe invention may be practiced without these details. One skilled in theart will recognize that embodiments of the present invention, some ofwhich are described below, may be incorporated into a number ofdifferent systems and devices including cameras, scanners, printers,computers, facsimile machine, mobile devices, including those deviceswith a display or camera capabilities, multimedia devices, and the like.The embodiments of the present invention may be implemented in software,hardware, firmware, or combinations thereof.

Components, or modules, shown in block diagrams are illustrative ofexemplary embodiments of the invention and are meant to avoid obscuringthe invention. It shall also be understood that throughout thisdiscussion that components may be described as separate functionalunits, which may comprise sub-units, but those skilled in the art willrecognize that the various components, or portions thereof, may bedivided into separate components or may be integrated together,including integrated within a single system or component.

Furthermore, connections between components/modules within the figuresare not intended to be limited to direct connections. Rather, databetween these components may be modified, re-formatted, or otherwisechanged by intermediary components. Also, additional or fewerconnections may be used. It shall also be noted that the terms “coupled”or “communicatively coupled” shall be understood to include directconnections, indirect connections through one or more intermediarydevices, and wireless connections.

Reference in the specification to “one embodiment,” “an embodiment,” or“embodiments,” means that a particular feature, structure,characteristic, or function described in connection with the embodimentor embodiments is included in at least one embodiment of the inventionand may be in more than one embodiment. The appearances of the phrase“in one embodiment,” or “in an embodiment,” or “in embodiments,” invarious places in the specification are not necessarily all referring tothe same embodiment or embodiments.

A. Overview

One specific application of the present invention is its use inembodiments of face modeling systems for face image understanding andprocessing. One skilled in the art shall recognize the generalapplicability of the present invention to modeling generally and toprocessing of object images other than face images.

Understanding human face images is important in visual communicationsystems. Key tasks in understanding human face images involve thepreservation of facial expressions and their evolution. The alignment offacial features (such as, for example, eye corners, face side, and lipcontours) often is an important component of such tasks. Facial featuresmay be used for enhancing or reconstructing detailed facial appearance,and also may be used for estimating the three-dimensional (3D) faceshapes and poses that comprise face image analysis and adjustment (e.g.,adjusting for self-occlusion, warping across poses for gaze correction,and un-distorting the perspective effects).

Traditional image filtering methods for image sharpening or enhancementare not optimal solutions for human face image understanding tasksbecause these methods do not take into account the inherent propertiesof the human face shapes and appearance. Recently, model-based methodshave been proposed for understanding human face images. One model-basedmethod that is widely used for face image analysis is based on atwo-dimensional (2D) Active Appearance Model (AAM). An AAM is astatistical model usually learned from a database of face imagescovering different properties of interest (e.g., different identities,different viewing angles, different lighting conditions, and differentexpressions). An AAM is a type of deformable model that typicallyconsists of two individual statistical models, one covering theappearance variation and another covering the shape deformation. An AAMmay be used to simultaneously describe the typical shape and appearancevariations (deformations) across human faces by training the model on aset of many face images exemplifying those variations (deformations).

A model-based method for modeling facial appearance variations and 3Dshape deformations may be based on a three-dimensional (3D) MorphableModel. The 3D Morphable Model is a statistical model that is typicallylearned from dense 3D scanned facial data under various situations. Amore efficient model-based method may be based on a 2D+3D AAM, whichuses the same sparse set of control points on the face that define a 2DAAM, but the 2D+3D AAM can model both 2D and 3D face shape deformationssimultaneously. This type of model enables estimation of 3D face poses,reconstruction of detailed facial deformations, and may also enablehandling of other factors such as, for example, self-occlusions.

The previously described models typically are global models thatconsider the variations of all the pixels on the face simultaneously. Aglobal model ensures the stability of the relationships among thecomponents in alignment and reconstruction by providing a globalconstraint on image variance of the facial components. However, a globalconstraint may limit the flexibility of the model. Since statisticalface models are usually developed by applying Principal ComponentAnalysis (PCA) to a set of training face images and keeping the mostdominant modes, some significant variations of small facial components,such as, for example, eyes and mouth, are often considered to be subtlevariations in the global models and thus abandoned. As a result, theglobal models often lose these subtle but important details in thereconstructed face images.

Embodiments of the present invention employ a 3D AAM face model that isbased on a combination of a set of individual appearance models offacial subcomponents and that has shape deformations constrained by aglobal 3D shape model. The set of individual appearance models ensuresincreased model flexibility for reconstruction of detailed face imagevariations while the global shape constraint ensures the structuralstability of the facial components.

One application of using such a model is in video communications, asillustrated in FIG. 1. A video communications system may comprise asender 105 that transmits a video stream of face images across a channel130 to a receiver 110. The received video stream may contain missingdata, and/or it may contain artifacts such as, for example, compressionartifacts. In a first embodiment 100 a, the receiver 110 may performface reconstruction based on a model 115 and parameters 120 (e.g., shapeparameters and texture parameters) that may be transmitted through thevideo stream. In alternate embodiments 100 b, the model 115 may beshared by both the sender 105 and the receiver 110, so that facereconstruction by the receiver 110 is based only on transmittedparameters 120. In yet other alternate embodiments 100 c, the receiver110 may fit an AAM to each received frame in order to detect faces andreconstruct face images without any model 115 or parameters 120 beingtransmitted.

B. System Implementations

FIG. 2 illustrates an object modeling system 200 according to variousembodiments of the invention. Object modeling system 200 comprises anobject model builder 210 that constructs an object model from a trainingdata set of object images 205, and an object model fitter 220 thatgenerates a reconstructed object image 225 by fitting an input objectimage 215 to a generated object model. In embodiments, object modelingsystem 200 may be a face modeling system, and the reconstructed face 225may be used for any of a number of purposes, including, withoutlimitation, to detect input face image 215 features such as, forexample, shapes, poses, eyes, mouth, etc.

1. Object Model Builder Implementations

The following sections describe various embodiments of implementationsof an object model builder 210 that generates an object model fittingsystem 345 based on a training data set of object images 205.

a) A First Embodiment of an Object Model Builder Implementation

FIG. 3 depicts a first embodiment of object model builder 210 thatcomprises mesh generator 310, 3D shape reconstructor and modeler 320,and subcomponent appearance modeler 330.

In embodiments, object model builder 210 may be a face model builder, atraining data set 205 may comprise face images, and a set of features orcontrol points may be identified and labeled in each face image withinthe training data set 205. In embodiments, the set of control points foran image may be previously identified and assigned labels or may beidentified and assigned labels by a user. Groups of these control pointsmay correspond to facial components such as, for example, eyes, brows,nose, mouth, and face sides. In embodiments, at least a subset of thecontrol points may be used to represent a 2D shape of a face image.Registration of the 2D face shapes (alignment of images to compensatefor camera movement) within the training data set 205 may eliminate therigid transformation and preserve the pure shape deformations in theface shapes. In embodiments, registration is implemented by applyingdirect factorization, although those skilled in the art will recognizethat many methods exist for performing registration and that the choiceof registration method is not critical to the invention.

(i) Mesh Generation and Shape Modeling

In embodiments, mesh generator 310 produces a mesh representation of anobject image within the training data set of object images 205 based onidentified control points of the image. FIG. 4 is an exemplaryillustration of using Delaunay triangulation to generate a mesh of aface image according to various embodiments of the invention. A set ofcontrol points are labeled (e.g., point 12 (425)) in a face image 420,and the labeled set of points provides the basis for subdividing theface image into a set of triangular regions 440. The boundaries of theset of triangular regions 440 form a mesh that may be used to representa 2D shape of the face image features. Those skilled in the art willrecognize that a variety of methods exist for generating a meshrepresentation of an object image, and that the choice of a meshgenerating method is not critical to the invention.

In embodiments, 3D shape reconstructor and modeler 320 receives a set ofmeshes representing 2D shapes and generates a reconstructed 3D shapemodel based on the set of meshes. In embodiments, a set of 2D shapes areobtained from the labeled landmarks (feature points) in the trainingdata set 205 by performing a shape alignment analysis, such as aProcrustes analysis, on the shapes in the training data set 205. Byperforming such analysis, the translational, rotational and scalingcomponents of the shapes can be reduced.

In embodiments, the comprehensive global 3D shape model can be derivedusing the set of 2D shapes as input to a non-rigid structure from motion(NRSFM) algorithm, wherein the 2D shape are reconstructed as projectionsof the 3D shapes. In embodiments, a NRSFM algorithm is applied to theset of 2D shapes to decompose the 3D shape bases. The NRSFM algorithmdirectly factors the 2D shapes into projection matrices and 3D shapebases. The algorithm is typically initialized by decomposing a matrix Wof the 2D shapes, into two matrices, M and B, via singular valuedecomposition (SVD), and solving for an ambiguity matrix, G, usingrotation and basis constraints using the equation:W=MGG⁻¹B   (1)

Once G is known, the projection matrices are MG, and the 3D shape basesare G⁻¹B.

In embodiments, a set of reconstructed 3D shapes corresponding to theset of input meshes is generated using the 3D shape bases projected ontoeach of the input meshes. The reconstructed 3D shape model is generatedby applying PCA to the set of reconstructed 3D shapes in order tocompute another set of 3D shape modes.

In embodiments, a 3D shape model S may be described by the equation:

$\begin{matrix}{S = {P \cdot \left( {B_{0} + {\sum\limits_{i = 1}^{N_{B}}{p_{i}B_{i}}}} \right)}} & (2)\end{matrix}$

where P is a camera projection matrix, B₀ is the mean shape, B_(i) is aset of N_(B) 3D shape modes, and p_(i) is a set of shape basisprojection weights. In embodiments, an iterative bilinear solution withNRSFM is used to estimate B_(i) and p_(i). In one embodiment, only thefirst few (1˜3, for example) dominant B_(i) along with the correspondingP and p_(i) are found in the NRSFM algorithm. The known B_(i), P, andp_(i) are then used to initialize the bilinear algorithm and solve forthe remaining B_(i), P, and p_(i) iteratively and sequentially until allparameters have been determined.

FIG. 5 illustrates the mean shape B₀ 510 and the first two 3D shapemodes B₁ 520 and B₂ 530 of an exemplary 3D face shape model derived froma set of 2D shapes 500 according to various embodiments of theinvention. The arrows (e.g., 525 and 535) depicted in the twoillustrated shape modes (520 and 530) describe the shape motion.

(a) Adding Symmetry Constraints to a 3D Object Shape Model

In embodiments, the accuracy of the 3D face shape model may be improvedby adding one or more symmetry constraints as in, for example,embodiments of method 1600.

In embodiments, a symmetry constraint may be added to the 3D Z-motion(depth) during the bilinear algorithm after the NRSFM algorithm has beenapplied. The bilinear algorithm iteratively solves for the remainingB_(i), P, and p_(i) by solving for B_(i) while keeping P and p_(i)constant, and then solving for P and p_(i) keeping B_(i) constant. Thisprocess alternates until the convergence criterion is achieved. Full orpartial motion symmetry may be enforced by adding specifically designedconstraints each iteration when solving for B_(i).

FIG. 6 illustrates views of two reconstructed face images (600 is thefrontal view of both reconstructed images) generated by fitting an inputface image to two 3D face models derived from the same training data setof images. The reconstructed image with the illustrated profile, bottomup, and top down views on the left (610 a, 620 a, and 630 arespectively) was fitted to a shape model that does not include symmetryconstraints, while the reconstructed image with the illustrated profile,bottom up, and top down views on the right (610 b, 620 b, and 630 brespectively) was fitted to a shape model that includes symmetryconstraints. Notice that the model with symmetry constraints produced amore realistic face image as seen most notably when comparingunconstrained image 620 a with constrained image 620 b.

(ii) Appearance Modeling

In embodiments, subcomponent appearance modeler 330 receives a trainingdata set of images 205 and generates a 2D image appearance modelcomprising a set of appearance models of image subcomponents.

In embodiments, a mean shape for object images in the training data set205 may be obtained by performing a shape alignment analysis, such as aProcrustes analysis, on the object images in the training data set 205.In embodiments, each object image in the training data set 205 is warpedto the mean shape in order to reduce size, alignment, and translationdifferences among the object images. Those skilled in the art willrecognize that the Procrustes analysis is one of a variety of shapealignment analysis methods that exist, and that the choice of a shapealignment analysis method is not critical to the invention. Inembodiments, each subcomponent i of a warped image is defined as a pixelor set of pixels forming a subset of the pixels in the shape normalizedtexture (i.e. canonical texture). In embodiments, the shape normalizedtexture may be manually defined, and, in other embodiments, may bedefined by the mean 2D shape. In embodiments, face textures may bewarped to this shape normalized texture to ensure pixel-to-pixelcorrespondence across textures so that the same pixel or set of pixelscomprising a subcomponent in the shape normalized texture represent thesame region of the face even though the original face textures may haveoriginally had different sizes, rotations, and positions in the trainingimages. As discussed below, a subcomponent appearance model I_(i) may becomputed from the pixels within the normalized subcomponent.

In embodiments, the triangular regions in the exemplary subdivided faceimage 440 may be grouped into larger regions representing facialsubcomponents (e.g., left eye, right eye, nose, brows, mouth, and facesides). An appearance model I_(i) of a the i_(th) subcomponent may bederived from a training data set of images by extracting a set of linearappearance bases capable of representing the i_(th) subcomponent in allthe training data set images set by using a method of generatingorthogonal bases (e.g., Principal Component Analysis (PCA)) andgenerating a linear combination of the bases according to the equation:

$\begin{matrix}{I_{i} = {A_{i\; 0} + {\sum\limits_{j = 1}^{N_{A_{i}}}{\lambda_{ij} \cdot A_{ij}}}}} & (3)\end{matrix}$

where I_(i) is the appearance model of subcomponent i, A_(i0) is themean subcomponent appearance image of subcomponent i, and A_(ij) andλ_(ij) are the j_(th) appearance basis and corresponding projectionweight, respectively, of the i_(th) subcomponent for all N_(Ai) bases.In various embodiments, the i_(th) subcomponent may be the global faceimage. This global image appearance model may be used to model theco-existence of the image subcomponents. The global model covers thevariations across all the image subcomponents together so it is able tosmooth boundaries between adjacent subcomponents which may otherwiseappear too distinct in a reconstructed image. In embodiments, the set ofappearance bases are ortho-normalized when one of the subcomponents is aglobal image appearance model.

In embodiments, a face appearance model comprising a set of subcomponentappearance models may be derived from a training data set of face images205. A set of N_(D) facial subcomponents is determined, and a set ofappearance models is generated as described in Equation (3).

(iii) Comprehensive Object Model Generation

In embodiments, object model builder 210 may generate a comprehensiveshape-normalized face model that is a 3D AAM, which is a face imageappearance model constrained by the global 3D shape model in Equation(2). In embodiments, the face image appearance model of the 3D AMMcomprises a set of subcomponent appearance models as described inEquation (3). A face image appearance model, I, may be expressed in theequation:

$\begin{matrix}{I = {\sum\limits_{i = 1}^{N_{D}}\left( {A_{i\; 0} + {\sum\limits_{j = 1}^{N_{Ai}}{\lambda_{ij}A_{ij}}}} \right)}} & (4)\end{matrix}$

where N_(D) is the number of subcomponents, N_(Ai) is the number ofappearance bases for the i_(th) subcomponent, and λ_(ij) are the basisprojection weights.

In embodiments, object model builder 210 may generate an object modelfitting system 345 comprising the 3D AAM comprising the set ofsubcomponent appearance models in Equation (4) constrained by the global3D shape model in Equation (2).

b) A Second Embodiment of an Object Model Builder Implementation

FIG. 7 depicts a second embodiment of object model builder 210 thatcomprises mesh generator 310; 3D shape reconstructor and modeler 320,subcomponent appearance modeler 330, and model balancer 710.

(i) Model Balancing

A training data set 205 of images may be unbalanced, and unbalancedtraining data may lead to biased eigenvectors for shape and/orappearance after generating orthonormal bases, such as by using PCA.Examples of an unbalanced training data set include a data set withproportionately greater representations of images of a particular type(e.g., images of a particular subject in a set of face images), ofimages having a particular pose, or of images associated with particularenvironmental conditions (e.g., lighting conditions).

In embodiments, model balancer 710 improves the predictive power ofshape and/or appearance models by correcting training data setimbalances. A training data set imbalance may be identified by fittingan interim object model to the training data set and calculating fittingerrors for the training images in the training data set. If a subset ofthe training images has fitting errors greater than a threshold, thetraining data set is determined to be unbalanced. A subset may bedefined as any number of images.

In embodiments, a training data set 205 of face images may be balancedeither by adding more of the images having fitting errors greater thanan error threshold to the data set (and increasing the size of the dataset), or by weighting the images having fitting errors greater than anerror threshold (and keeping the size of the data set constant). Inembodiments, weights associated with the overall model fitting error maybe computed by combining the shape registration error and the appearanceerror for an image. For example, in an embodiment, if there are threetraining images with fitting errors [1000, 2000, 3000], the weightsassigned to the images may be computed by dividing the fitting errors bythe total registration error, resulting in weights of [⅙, ⅓, ½] for thethree training images respectively. Similarly, all fitting errors can bedivided by the minimum fitting error and the result can be rounded toget a new set of numbers, [1000, 2000, 3000]/1000=[1 2 3]. These numberscorrespond to the number of copies of each image to be included in thetraining data set. In other embodiments, the number of examples of eachtype of training image within a data set may be used to perform a singlerebalancing of the training data by repeating training samples so thatthere is an equal number of each type of image within the draining dataset. For example, in an embodiment, if there are two training images oftype A and one of type B, image B can be repeated in the training dataset so that the training data consists of the two original type Aimages, 1 original type B image, and 1 repeated type B image. Thus,images with larger fitting errors or fewer examples are weighted more.

In embodiments, a more balanced interim object model may be generated byapplying PCA to the balanced training data set. In various embodiments,the shape and/or appearance models may be weighted and re-trained.

c) A Third Embodiment of an Object Model Builder Implementation

FIG. 8 depicts a third embodiment of object model builder 210 thatcomprises mesh generator 310, 3D shape reconstructor and modeler 320,subcomponent appearance modeler 330, and manifold builder 810. Theobject model fitting system 345 may further comprise shape and/ortexture manifolds.

(i) Shape and Texture Manifold Building

As the size of the training data set increases, the variability of theshape and appearance model parameters also typically increases. Thisincreased parameter variability may increase the likelihood ofgeneration of a non-valid reconstructed image during model fitting. Inembodiments, manifold builder 810 generates a set of shape parametervectors and/or a set of texture parameter vectors by fitting thegenerated shape and appearance models to the images in the training dataset 205 in order to find the shape and texture parameters that best fitthe models. In embodiments, a shape parameter vector is generated fromthe shape parameters for each training data set image, and a textureparameter vector is generated from the texture parameters for eachtraining data set image. Manifold builder 810 builds a shape manifoldusing the set of generated shape parameter vectors, and/or builds atexture manifold using the set of generated texture parameter vectors.

In embodiments, the parameter vectors within a manifold are clusteredaccording to a clustering algorithm, such as, for example, K-MeansClustering (K-Means). Those skilled in the art will recognize that thereare many clustering algorithms (Mean Shift is another exemplaryclustering algorithm), and that the choice of a clustering algorithm isnot critical to the invention. The cluster centers and respectivecluster co-variances determined by the clustering algorithm provide thebasis for manifold constraints to be added to the object model fittingsystem 345. During model fitting, the likelihood of an invalidreconstructed image 225 generated from the object model fitting system345 can be reduced because the updating of shape and/or textureparameter vectors may be constrained based on the distance of thesevectors to manifold cluster centers. Shape and texture manifold buildingmay be implemented by manifold builder 810 as described in embodimentsof method 2100.

FIG. 9 illustrates exemplary reconstructed face shapes derived fromfitting an input face image to two face shape models according tovarious embodiments of the invention. Reconstructed face shape 910 wasfitted to a face shape without including manifold constraints, whilereconstructed face shape 920 was fitted to a model of the same faceshape that included manifold constraints. Shape 920 produces a morerealistic face shape as compared with the unconstrained shape 910.

d) A Fourth Embodiment of an Object Model Builder Implementation

FIG. 10 depicts a fourth embodiment of object model builder 210 thatcomprises mesh generator 310, 3D shape reconstructor and modeler 320,subcomponent appearance modeler 330, model balancer 710, and manifoldbuilder 810. This embodiment combines all of the elements of theprevious object model builder 210 embodiments, and may generate anobject model fitting system 345 comprising a 3D AAM comprising a set ofsubcomponent appearance models, a global 3D shape model, and shapeand/or texture manifolds.

2. Object Model Fitting Implementations

FIG. 11 depicts an object model fitter 220 that generates areconstructed image 225 by using an object model fitting system 345 tofit an object model to an input object image 215 according to variousembodiments of the invention. Object model fitter 220 comprisesreconstructed image estimator 1110 and reconstructed image updater 1120.

In embodiments, object model fitter 220 is a face model fitter, andreconstructed image estimator 1110 uses an object model fitting system345 comprising a face model to fit an input face image 215 to the facemodel by determining the values of parameters corresponding to 3D facepose, face shape, and facial features of the input face image 215 thatminimize the difference between the face model and the input face image215. In embodiments, the face model may be the appearance model inEquation (4) constrained by the global 3D shape model in Equation (2).In embodiments, minimization is computed by iteratively updatingparameter values within a fitting function, re-computing a face modelestimate, and re-fitting the input face image to the current face modelestimate using the Levenberg-Marquardt (LM) algorithm, a variation ofthe steepest (gradient) descent algorithm, which is well known to thoseskilled in the art. FIG. 12 depicts an exemplary face model fittingprocess according to various embodiments of the invention.

The current reconstructed estimate of 3D face shape and pose 1205 isprojected onto the input face image 1210, and the projected input faceimage is warped to a canonical view 1235. In the illustrated example,the frontal view is the canonical view. The current reconstructed faceappearance estimate in canonical view 1230 is compared to the warpedinput face image 1235, and the difference is a fitting error. Inembodiments, the fitting error may comprise a set of fitting errors.

In embodiments, reconstructed object updater 1120 computes a fittingerror representing the difference between a current object modelappearance estimate in a canonical view 1230 and a current appearanceestimate of an input object image warped to the canonical view 1235based on a current estimate of 3D object shape and pose 1205.

In embodiments, reconstructed object updater 1120 iteratively applies afitting function 1215 for determining the model shape (p), pose (P), andtexture (λ) parameter 1220 values that minimize the fitting error. Inembodiments, a fitting function 1215 may be implemented according to theequation:

$\begin{matrix}{\min\limits_{\lambda,p,P}{W\left\lbrack {{\sum\limits_{i = 1}^{N_{D}}\left( {A_{i\; 0} + {\sum\limits_{j = 1}^{N_{Ai}}{\lambda_{ij}A_{ij}}}} \right)} - {T\left( {{I;p},P} \right)}} \right\rbrack}^{2}} & (5)\end{matrix}$

where W is a weighting matrix and T(I;p,P) is a transformation of theimage pixels, I, to the mean shape using the shape and pose parameters.

Initially, mean values for reconstructed object image 225 3D shape (p),pose (P), and texture (λ) parameters 1220 are used. Values for 3D shape(p), pose (P), and texture (λ) parameters 1220 are iteratively adjustedto reduce the fitting error. The final values for 3D shape (p), pose(P), and texture (λ) parameters 1220 are determined when the fittingerror between the set of facial appearance models and the projectedinput object image is minimized, or when some other stopping criterionhas been reached, such as, for example, number of iterations or lack ofsignificant changes in fitting error between iterations.

a) Inverse Compositional Model Fitting Solution

In embodiments, the final values for 3D shape (p), pose (P), and texture(λ) parameters 1220 may be computed using an inverse compositionalsolution fitting function 1215 that determines reconstructed texture (λ)parameters based on solving for the minimized update to the shapeparameters Δp in the image frame according to the equation:

$\begin{matrix}{\min\limits_{\lambda,{\Delta\; p}}{W{{{T\left( {{A(\lambda)};{\Delta\; p}} \right)} - {T\left( {I;p} \right)}}}^{2}}} & (6)\end{matrix}$

and then computes texture reconstruction A(λ) according to the equation:

$\begin{matrix}{{A(\lambda)} = {A_{0} + {\sum\limits_{i = 1}^{N_{a}}{A\;\lambda}}}} & (7)\end{matrix}$

where A₀ is the mean appearance and N_(a) is the number of appearancebases.

In embodiments, the update to the shape parameters Δp may be calculatedaccording to the equation:Δp=R·E   (8)

where R is a shape parameter update matrix and E is a registration errorvector E=(A₀−I).

In embodiments, Δp is used to determine S_(shape), the new shapeestimate, according to the equationS _(shape) =T(S ₀ ;p)+T(S ₀ ;−Δp)   (9)

where S_(shape) is the mean shape (S₀) warped by the shape parameterupdate, Δp, and then warped to the image frame by the current shapeparameters, p, T(S₀;p) is the current shape estimate, and T(S₀;−Δp) isthe delta shape (Note that T(x;p) defines a warping of the shape, x, bythe parameters, p).

In embodiments, a 2D shape update Δ{circumflex over (p)} in the imageframe may be determined according to the equations:

$\begin{matrix}{\min\limits_{\Delta\;\hat{p}}{{S_{shape} - \left( {S_{o} + {S\left( {p - {\Delta\;\hat{p}}} \right)}} \right)}}^{2}} & (10) \\{{\Delta\;\hat{p}} = {- {S^{\prime}\left( {S_{shape} - \left( {S_{0} + {Sp}} \right)} \right)}}} & (11)\end{matrix}$

where S is the set of shape eigenvectors, S′ is the transpose of S, andS_(p) is the modified set of shape eigenvectors. The updated shapeparameters p in the image frame may be computed using the previous bestparameters (where best parameters are the parameters from the previousiteration that resulted in the lowest registration error), p₀ (the meanshape parameters) and Δ{circumflex over (p)}, according to the equation:p=p ₀ −Δ{circumflex over (p)}  (12)

Note that the solution for shape reconstruction in Equations (10) and(11) does not include an implementation of a weighting matrix W for theshape vectors. In embodiments, a shape reconstruction may be weighted,and Δ{circumflex over (p)} in the image frame may be determinedaccording to the equation:Δ{circumflex over (p)}=−(S′WS)¹ S′W(S _(shape)−(S ₀ +Sp))   (13)

In embodiments, it should be noted that a 3D shape update Δp may berepresented as

$\min\limits_{\Delta\; p}{{S_{shape} - {P\left( {S_{o} + {S\left( {p - {\Delta\;\hat{p}}} \right)}} \right)}}}^{2}$

where P is the projection matrix. The solution to this equation does nothave a closed form (like Equation (11) for the 2D case). In a 3D system,an approximate solution for P and Δp is determined by an iterativegradient descent algorithm.

In embodiments, the image texture λ is extracted from the image framewarped using the updated shape parameters (T(I,p)), according to theequations:

$\begin{matrix}{\min\limits_{\lambda}{W{{A_{o} + {A\;\lambda} - {T\left( {I;p} \right)}}}^{2}}} & (14) \\{\lambda = {{- \left( {A^{\prime}{WA}} \right)^{- 1}}A^{\prime}{W\left( {A_{o} - {T\left( {I;p} \right)}} \right)}}} & (15)\end{matrix}$

where A is the set of texture eigenvectors, A₀ is the set of meantexture vectors, the transposed texture vectors matrix A′ is a P_(T)×Nmatrix of texture eigenvectors (P_(T) is the number of texture modes andN is the number of pixels), and W is an N×N weighting matrix.

In embodiments, the weighted reconstruction error matrix E then may bedetermined based on the difference between the reconstructed image I andthe texture λ extracted from the image according to the equation:

$\begin{matrix}{E = {A_{0} + {\sum\limits_{i = 1}^{N_{Ai}}{A\;\lambda}} - I}} & (16)\end{matrix}$

where N_(Ai) is the number of texture bases for the i_(th) subcomponent.

b) Robust Model Fitting

The illustrated exemplary face appearance model estimate 1230 and warpedinput face image appearance estimate 1235 have been subdivided into sixfacial subcomponents (left and right eyes, nose, mouth, forehead/browline, and cheeks/chin area). In embodiments, after fitting the currentface model appearance estimate to the input face image appearanceestimate, face subdivisions (such as, for example, facial subcomponents)and/or pixels may be weighted related to one or more factors. Inembodiments, the object model fitting system 345 may further compriseweights.

In embodiments, weighting may be associated with image characteristics,such as, for example, model complexities, fitting errors, gradientmagnitude, edges, and categories of persons and/or capturing conditions.The type of weighting method implemented may depend on the model and/orfitting objective. For example, weights based on registration errors maybe useful for fitting the model given occlusions in the input objectimage. In another example, weights based on the 3D object pose may beused to downweight parts of the object model that face away from thecamera and/or increasing the weights of object model parts most directlyfacing the camera. In various embodiments, the type of weighting methodimplemented may be based on both the registration error and 3D pose.Those skilled in the art will recognize that various types of weightingmethods exist, and that the selection of weighting method is notcritical to aspects of the invention.

In embodiments, robust fitting may be implemented by applying aweighting matrix 1225 based on different weighting of features ofpixels, regions of pixels, and/or facial subcomponent regions of pixels.For example, in an embodiment, a face model may be sub-divided into aset of triangular regions (triangles), and weighting may be applied tothe triangles within the set of triangles. Each triangle contains anumber of pixels, and the registration error for each triangle may becomputed by finding the mean of the absolute values of the registrationerrors for each pixel in the triangle. The mean registration errors forthe set of triangles are computed and converted to binary weights duringeach iteration of the model fitting process. Binary weights are computedbased on the distribution of the set of triangle registration errors.For example, the median of the registration errors for the set oftriangles may be used to define a Gaussian function. For example, if atriangle registration error is less than an empirically estimatedthreshold of 0.5, the triangle is given a binary weight of zero.Otherwise, the triangle is given a binary weight of one.

Typically, fitting robustness increases and efficiency decreases as thesize of a weighted image region (such as, for example, a triangle or afacial subcomponent) decreases, but compromises may be made to increasefitting efficiency. A binary weighting function (such as, for example,as previously described in the exemplary scenario) has the advantage ofefficiency, but at the cost of accuracy compared to other popularweighting functions, such as those based on exponential or Gaussiandistributions. Those skilled in the art will recognize that the choiceof weighting function for implementation of robust fitting is notcritical to aspects of the invention.

In various embodiments, weighting at the subdivision and/or pixel levelmay be based on an image gradient map. For example, edge pixels may begiven higher weights because edges contain useful information aboutmodel shape and are immune to variations in most lighting conditions. Inembodiments, pixel weighting may be based on characteristics such as,for example, image error, distance from the model center, and gradientmagnitude.

In embodiments, pose-based pixel weighting may be applied so that higherweights are given to pixels most directly facing the camera. FIG. 13illustrates an exemplary computation of pose-based weighting of pixelswithin subdivisions of a face image according to various embodiments ofthe invention. The exemplary input face image 1305 has been subdividedinto triangular regions using Delaunay Triangulation, as previouslydescribed, and the camera axis 1315 (relative position of the camera1310) for each triangle is diagrammed. In embodiments, the normal vector1325 for each triangle is computed, and the angle θ 1320 between thenormal vector and the camera axis is computed. In embodiments,pose-based model weights for the triangular regions may be based on adistribution of the computed angle θ of the normal vectors according tothe equation:1−(min(θ,π/2)*2/π)⁴   (17)

Because the pose and shape parameters and mesh define a 3D shape meshcomprising triangles/subdivisions, each triangle in the 3D shape meshmay have a normal direction. The angles between the normals and thecamera axis in the 3D shape mesh can be used to update the weights.Thus, pose and shape parameters affect the weights because they are usedto determine the 3D shape, but, in embodiments, the weights are computedbased on the computed angle θ of the normal vectors, as described. Inembodiments, pose-based weights may be updated by determining an anglefor a triangle. All pixels in the mesh triangle may be associated withthe same angle or a normal angle may be interpolated at each pixel inthe triangle. A subcomponent of the object can be formed from aplurality of triangles and/or portions thereof, and the pose-basedweight associated with the subcomponent may be computed by averaging theangles associated with the pixels that form the subcomponent. Forexample, in embodiments, the normal angle for a subcomponent may becomputed by averaging the angles associated with pixels that form thesubcomponent.

In embodiments, registration error weighting of object imagesubdivisions and/or pixels may enable robust fitting of the object modelto an input object image containing occlusions (regions with missingdata). FIG. 14 illustrates an exemplary weighting of pixels based onregistration error within an occluded face image according to variousembodiments of the invention. The exemplary input face image 1410 hasbeen subdivided into triangular regions 1420 using DelaunayTriangulation, as previously described. In embodiments, the triangularregions (e.g., 1422 and 1424) within a facial subcomponent correspondingto the occluded right eye 1412 of an input face image may have largerfitting errors, and thus those regions may be given smaller weights thanother regions (e.g., 1426) within the input face image. Since, inembodiments, weighting may be done at both a pixel level and asubcomponent level, the pixels in the occluded triangular regions havingthe greater fitting error (e.g., 1422) may be given lower weights thatthe pixels in other triangular regions (e.g., 1424) within the occludedregion. In the illustration, the regions within the occludedsubcomponent 1412 having the greater fitting error (e.g., 1422) arecolored darker than the other regions (e.g., 1424) within the occludedsubcomponent.

In embodiments, weights assigned based on registration errorautomatically adapt to extracted input face image data, so robustestimation policies should cover input face images with and withoutocclusions. As the model shape iterates towards convergence, the modelwill cover new pixels in the image. Each time the registration error isupdated, new weights are computed. If there are no occlusions theregistration error will likely be similar throughout the face region sothe weights will all be relatively equal. However, if a certain part ofthe model is unable to fit to a part of the face, due to occlusions orotherwise, that part of the face region will result in largeregistration errors which in turn will give that part of the model lowerweights.

(i) Iterative Weight Matrix Updating

In embodiments, the weighting matrix 1225 may be adjusted iteratively bythe reconstructed object updater 1120 during the fitting process. Forexample, in embodiments employing error-based weighting, the previousobject image registration error may be used to compute new fittingweights at the beginning of each iteration of the fitting process, andthen the object model parameters are updated.

In embodiments implementing an inverse compositional fitting solution, ashape parameter update Δp may be found by multiplying the shapeparameter update matrix R with a registration error weighting vector Eaccording to Equation (8), as previously discussed. In embodiments, theshape parameter update matrix R may be pre-computed and thus not updatedduring fitting if shape parameter weighting is not used (e.g., aweighting matrix W is set to be the identity matrix), or if constantshape parameter weighting is used (e.g., the weighting matrix W is setto be a constant weight). However, in other embodiments, a weightingmatrix W is used to compute R and the weighting matrix W must bere-computed at each fitting iteration.

For example, in embodiments, a shape parameter update Δp may be computedusing an R that is computed using a weighting matrix W according to theequation:Δp=−(SD′WSD)⁻¹ SD′W(A ₀ −I)   (18)

where R=−(SD′WSD)⁻¹SD′W and E=(A₀−I) and SD is a P_(S)×N steepestdescent matrix of shape eigenvectors (P_(S) is the number of shape modesand N is the number of pixels) to a gradient of appearance meansaccording to the equation:

$\begin{matrix}{{SD} = \left( {{\nabla A_{0}}\frac{\partial W}{\partial p}} \right)} & (19)\end{matrix}$

In this case, the objective function for the shape update Δp isexpressed in terms of the steepest descent matrix and W (an N×Nweighting matrix) according to the equation:

$\begin{matrix}{\min\limits_{\Delta\; p}{W{{A_{0} + {{\nabla A_{0}}\frac{\partial W}{\partial p}\Delta\; p} - I}}^{2}}} & (20)\end{matrix}$

where A₀ is the mean shape, and I is the current shape model estimate.

In embodiments, an N×N weighting matrix W also may be applied to theextraction of texture parameters λ from the image frame, as described inEquations (14) and (15).

In embodiments, an iterative update to weighting matrix W is computedbased on the image reconstruction error, E (See Equation (16)) using thereconstructed image, A₀+Aλ. Note that λ must first be estimated (SeeEquations (14) and (15)) before the reconstruction error E can becomputed, as previously described.

Iterative fitting methods employing pixel based error weighting, asillustrated in the previous examples, may be computationally expensivebecause the weights change for every iteration and thus the parameterupdate matrices R (e.g., −(SD′WSD)⁻¹SD′W in Equation (18) and−(A′WA)⁻¹A′W in Equation (15)) must be recomputed for every pixel atevery iteration. In embodiments, the performance of iterative fittingmethods may be optimized by solving for model parameter updates based onthe weights of groups of pixels within regions (such as, for example,the triangular regions illustrated in FIG. 14) of an image. Inembodiments, these regions may correspond to image subcomponents.

For example, in embodiments, the image registration error of the pixelsmay be used to compute the mean error e for each triangular region(triangle) of an image subdivided into t triangular regions of pixels(e.g., See 440 in FIG. 4):E _(t) =[e ₁ , e ₂ , . . . e _(t)] (mean triangle errors, for ttriangles)

The median error E_(m) may be computed as the center of a Gaussiandistribution G(x) of the set of t triangle errors:E _(m)=median(E _(t))

As is well known to those skilled in the art, the standard deviationstddev of this distribution may be computed as a number multiplied bythe median. For example, using a median absolute deviation, the standarddeviation can be determined as stddev=1.4826*median. Although, oneskilled in the art shall recognize that other alternatives to the medianabsolute deviation may be used. The Gaussian function G(x) may becomputed as:G(x)=exp(−0.5*(x−E _(m))²/(1.4826*E _(m))²)

In embodiments, the weight w_(i) of the i_(th) triangle may be computedbased on the value of the mean triangle error e_(i) of the i^(th)triangle on this distribution:w _(i) =G(e _(i))

In alternate embodiments, the weight w_(i) of the i^(th) triangle may bea binary weight based on the Gaussian function G(x) such as, forexample:If w_(i)<0.5, then w_(i)=0 else w_(i)=1

where 0.5 is an arbitrary threshold between 0 and 1.

In embodiments, weights representing a region-based level of granularitymay be used during fitting to solve for updated texture (λ) and shapeparameter updates (Δp) of an image. For example, given an imagecontaining T components (triangles) where a triangle t contains a set ofx_(t) pixels, a N×P_(T) texture eigenvectors error weighting matrix Aand a N×P_(S) shape eigenvectors error weighting matrix SD are computedfor each triangle t, where A_(x) is a 1×P_(T) row of the eigenvectormatrix and SD_(x) is a 1×P_(S) row of the steepest descent matrix.Solutions for the updated image texture λ and shape Δp parameters usingtriangle-based weights may be implemented according to the equations:

$\begin{matrix}{\lambda = {{- \left( {\sum\limits_{t = 1}^{T}{w_{t}{\sum\limits_{x \in x_{t}}{A_{x}^{\prime}A_{x}}}}} \right)^{- 1}}A^{\prime}{W\left( {A_{0} - I} \right)}}} & (21) \\{{\Delta\; p} = {{- \left( {\sum\limits_{t = 1}^{T}{w_{t}{\sum\limits_{x \in x_{t}}{{SD}_{x}^{\prime}{SD}_{x}}}}} \right)^{- 1}}{SD}^{\prime}{W\left( {A_{0} - I} \right)}}} & (22)\end{matrix}$

In embodiments, the values

$\sum\limits_{x \in x_{t}}{A_{x}^{\prime}A_{x}\mspace{14mu}{and}\mspace{14mu}{\sum\limits_{x \in x_{t}}{{SD}_{x}^{\prime}{SD}_{x}}}}$may be pre-computed in order to further optimize the fitting methodperformance.

c) Object Model Fitting with Manifold Constraints

In embodiments, the object model fitting system 345 may further comprisemanifold constraints. Shape and/or texture manifolds, which may havebeen generated in embodiments of manifold builder 810, may be used toconstrain parameter updates during the object model fitting process, asexemplified by method 2500.

C. Methods for Object Model Building

The following sections describe various embodiments of methods forgenerating an object model based on a training data set of images. Themethod embodiments may be implemented in embodiments of object modelbuilder 210.

1. A First Embodiment of a Method for Generating an Object Model

FIG. 15 depicts a method 1500 for generating an object model fittingsystem comprising an object model according to various embodiments ofthe invention. Method 1500 comprises the steps of generating a 3D shapemodel and an appearance model based on a set of 2D subcomponent models1505, and generating an object model fitting system comprising an objectmodel comprising the appearance model constrained by the 3D shape model1510. Method 1500 may be implemented in embodiments of object modelbuilder 210 comprising mesh generator 310, 3D shape reconstructor andmodeler 320, and subcomponent appearance modeler 330.

a) Generating 3D Shape and Appearance Models

In embodiments, a 3D shape model and an appearance model of an objectare generated 1505 based on a training data set of object images.

FIG. 16 depicts a method 1600 for extracting features from an objectimage according to various embodiments of the invention. In embodimentsof method 1500, step 1505 may comprise an implementation of method 1600.Method 1600 may be implemented in embodiments of mesh generator 310.

A set of features or control points for an object image may beidentified 1605 and assigned labels. Control points may be pre-definedas part of the set of object images, user defined, or both. Inembodiments, the training data set may comprise face images, and the setof control points may be labeled in each face image within the trainingdata set. Groups of these control points within a face image maycorrespond to facial components such as, for example, eyes, brows, nose,mouth, and face sides. The exemplary face image 420 illustrates labeledcontrol points.

In embodiments, the labeled set of control points may provide the basisfor subdividing 1510 the face image into a set of triangular regionsusing Delaunay triangulation, as illustrated in FIG. 4. For consistency,the set of triangular regions may be normalized 1615 in terms of sizeand camera angle. In embodiments, the canonical camera angle may be afrontal view. In embodiments, a global 2D shape model of an object imagemay be generated based on the set of control points 1620.

(i) Generating a 3D Shape Model

FIG. 17 depicts a method 1700 for generating a 3D shape and 3D shapemodes for an object according to various embodiments of the invention.In embodiments of method 1500, step 1505 may comprise an implementationof method 1700. Method 1700 may be implemented in embodiments of 3Dshape reconstructor and modeler 320.

In embodiments, a 3D shape model may be generated based on a set of 2Dshapes 1705 obtained from the meshes generated from the training dataset of object images. A 3D shape model based on the identified set ofcontrol points on the object (See exemplary FIG. 5) can model objectshape deformations. In embodiments, a non-rigid structure from motion(NRSFM) algorithm may be applied to the set of 2D shapes to generate1710 a set of reconstructed 3D shape modes. As previously described, aset of 2D shapes are obtained from the training data set 205 byperforming a shape alignment analysis, such as a Procrustes analysis, onthe object images in the training data set 205. In embodiments, PCA maybe applied 1715 to the reconstructed set of 3D shape vectors to generatea global object 3D shape model (See Equation (2)).

(a) Generating a Shape Model Having Symmetry Constraints

In embodiments, the accuracy of a 3D shape model may be improved byadding one or more symmetry constraints. For example, in an embodiment,a 3D z-motion (depth) constraint may be added as part of the NRSFM asdiscussed previously with respect to FIGS. 5 and 6. Symmetry is enforcedon the z dimension because this dimension typically is most sensitive tonoise but generally does not vary within an image. In embodiments, thesymmetry constraint is associated with particular characteristics of anobject shape. For example, in embodiments, face subcomponentscorresponding to the left eye and the right eye may be set to lie on thesame plane, or close to the same plane.

In various embodiments, a symmetry constraint may be applied to a set ofF shapes having N points per shape according to the following equation:

$\begin{matrix}{{\min\limits_{S}{{W - {P_{C}S}}}^{2}} + {\sum\limits_{i = 1}^{N}{K_{i}{\sum\limits_{j = 1}^{M}{{aSd}_{ij}}^{2}}}}} & (23)\end{matrix}$

where W is the matrix of 2D points (2F×N), S is the 3D shape eigenvector(3×N), PC are the scaled rotation matrices (2F×3), i is a facialsubcomponent (e.g. mouth corner), j is a pair of points within thefacial subcomponent i, K_(i) is a constraint weight, a is a symmetrydimension, such as a z-dimension (e.g. a=[0 0 1]), and d_(ij) is avector representing the pair of points it for which symmetry is beingenforced, for example:

$\begin{matrix}{d_{ij} = \begin{bmatrix}1 \\0 \\\vdots \\0 \\{- 1} \\0 \\\vdots \\0\end{bmatrix}} & (24)\end{matrix}$

FIG. 6 illustrates an exemplary application of symmetry constraintsduring 3D shape model generation, as previously discussed.

(ii) Generating an Appearance Model

In embodiments, a face appearance model comprising a set of subcomponentappearance models 1505 may be derived from a training data set of faceimages. Step 1505 may be implemented in embodiments of subcomponentappearance modeler 330.

In embodiments, the triangular regions may be grouped 1625 into largerregions representing facial subcomponents, such as, for example, theleft eye and the nose. FIG. 18 illustrates an exemplary subdivided faceimage 1800 with a right eye subcomponent 1805 comprising four triangularregions.

An appearance model of each facial subcomponent may be generated. Inembodiments, an appearance model I of a subcomponent i may be derivedfrom a training data set of images by extracting a set of linear texturebases capable of representing the subcomponent i in the training dataset of images by using Principal Component Analysis (PCA) or the likeand generating a linear combination of the bases according to Equation(3), reproduced below:

$\begin{matrix}{I_{i} = {A_{i\; 0} + {\sum\limits_{j = 1}^{N_{A_{i}}}{\lambda_{ij} \cdot A_{ij}}}}} & (3)\end{matrix}$

where I_(i) is the appearance model of subcomponent i, A_(i0) is themean subcomponent appearance image of subcomponent i, and A_(ij) andλ_(ij) are the j_(th) appearance basis and corresponding projectionweight, respectively, of the i_(th) subcomponent for all N_(Ai) bases.In various embodiments, the i_(th) subcomponent may be the global faceappearance model. This global face appearance model may be used to modelthe co-existence of the image subcomponents.

In embodiments, a face appearance model comprising a set of subcomponentappearance models may be derived from the training data set of faceimages. A set of N_(D) facial subcomponents is determined, and an N_(D)set of appearance models is generated as described in Equation (3).

b) Generating a Comprehensive Object Model

In embodiments, a comprehensive 3D AAM face model is generated 1510,comprising a face appearance model formed from a set of facesubcomponent appearance models (that may have been generated asdescribed in Equation (3)), which is constrained by a global 3D faceshape model (See Equation (2)). A face image appearance model, I, may beexpressed according to Equation (4), reproduced here:

$\begin{matrix}{I = {\sum\limits_{i = 1}^{N_{D}}\left( {A_{i\; 0} + {\sum\limits_{j = 1}^{N_{Ai}}{\lambda_{ij}A_{ij}}}} \right)}} & (4)\end{matrix}$

where N_(D) is the number of subcomponents, N_(Ai) is the number ofappearance bases for the i_(th) subcomponent, and λ_(ij) are the basisprojection weights.

In various embodiments, a subcomponent i may be a global face image.This global face appearance model may be used to model the co-existenceof the face subcomponents. The global model covers the variations acrossall the subcomponents together so it is able to smooth boundariesbetween adjacent subcomponents which may otherwise appear too distinctin a reconstructed face. In embodiments, the set of appearance bases areortho-normalized when one of the subcomponents is a global faceappearance model.

2. A Second Embodiment of a Method for Generating an Object Model

FIG. 19 depicts a method 1900 for generating a balanced object modelaccording to various embodiments of the invention. Method 1900 may beimplemented in embodiments of object model builder 210 comprising meshgenerator 310, 3D shape reconstructor and modeler 320, subcomponentappearance modeler 330, and model balancer 710.

a) Model Balancing

As discussed previously, a training data set of object images may beunbalanced, and unbalanced training data may lead to biased eigenvectorsfor shape and/or texture after PCA is performed. In embodiments, a 3Dshape model and an appearance model of an object both are generated 1905based on a training data set of object images, in the same way aspreviously described for step 1505 in method 1500.

In embodiments, shape and/or appearance models may be improved bycorrecting training data set imbalances. Identification and correctionof training data set imbalances may be implemented in embodiments ofmodel balancer 710.

In embodiments, an interim object model comprising the generatedappearance model constrained by the generated 3D shape model isgenerated 1910 in the same way as previously described for step 1510 inmethod 1500. A training data set imbalance may be identified by fittingthe interim object model to the training data set object images 1915 andcalculating fitting errors for the fitted object images in the trainingdata set. For example, in embodiments, after the n^(th) round of modelfitting, if the maximum change in error ΔE_(i,n) for the i^(th) image,max(ΔE_(i,n)), divided by the previous maximum error, max(E_(i,n−1)), isgreater than a threshold 1920, then the training set remains unbalanced.In embodiments, an exemplary threshold 1920 may be 1-5% of the maximumerror.

In embodiments, an unbalanced training data set of face images may bebalanced 1925 either by adding more of the face images having fittingerrors greater than the threshold to the data set (and increasing thesize of the data set), or by weighting the face images having fittingerrors greater than the threshold (and keeping the size of the data setconstant). In embodiments, weights associated with the overall modelfitting error may be computed by combining the shape registration errorand the appearance error for an object image. As previously described,in an embodiment, if there are three training images with fitting errors[1000, 2000, 3000], the weights assigned to the images may be computedby dividing the fitting errors by the total registration error,resulting in weights of [⅙, ⅓, ½] for the three training imagesrespectively. Similarly, all fitting errors can be divided by theminimum fitting error and the result can be rounded to get a new set ofnumbers, [1000, 2000, 3000]/1000=[1 2 3]. These numbers correspond tothe number of copies of each image to be included in the training dataset. In other embodiments, the number of examples of each type oftraining image within a data set may be used to perform a singlerebalancing of the training data by repeating training samples so thatthere is an equal number of each type of image within the draining dataset. For example, in an embodiment, if there are two training images oftype A and one of type B, image B can be repeated in the training dataset so that the training data consists of the two original type Aimages, 1 original type B image, and 1 repeated type B image. Thus,images with larger fitting errors or fewer examples are weighted more.

A new interim object model may be generated 1930 by applying PCA to thebalanced training data set, and steps 1915 and 1920 are repeated usingthe new, re-generated interim model. In various embodiments, the shapeand/or appearance models may be weighted and re-trained. In embodiments,if no images in the training data set have fitting errors greater thanthe threshold 1920, the interim object model is set as the object model1935.

3. A Third Embodiment of a Method for Generating an Object Model

FIG. 20 depicts a method 2000 for generating an object model fittingsystem comprising an object model and manifold constraints according tovarious embodiments of the invention. Method 2000 may be implemented inembodiments of object model builder 210 comprising mesh generator 310,3D shape reconstructor and modeler 320, subcomponent appearance modeler330, and manifold builder 810.

As previously discussed, the variability of the shape and appearancemodel parameters may increase as the size of the training data setincreases. This increased parameter variability may increase thelikelihood of generation of a non-valid reconstructed object imageduring model fitting when model parameters are updated. In embodiments,model parameter updating may be constrained based on manifolds of shapeand/or texture parameter vectors generated from the training data setobject images.

In embodiments, a 3D shape model and an appearance model of an objectboth are generated 2005 based on a training data set of object images,and an object model fitting system comprising an object model comprisingthe appearance model constrained by the 3D shape model is generated2010, both in the same way as previously described for steps 1505 and1510 in method 1500. Shape and/or texture manifolds are generated 2015,and are added 2020 as manifold constraints to the object model fittingsystem.

a) Generating Shape and Texture Manifolds

In embodiments, shape and texture manifolds are generated 2015 based onthe training data set of object images and the generated object model.Step 2015 may be implemented in embodiments of manifold builder 810.

FIG. 21 depicts a method 2100 for building shape and texture manifoldsaccording to various embodiments of the invention. Method 2100 may beimplemented in embodiments of step 2015 in method 2000.

In embodiments, a set of shape eigenvectors and a set of appearanceeigenvectors are generated by fitting the generated shape and appearancemodels to the images in the training data set 2005. A shape manifold isbuilt using vectors of shape parameters and a texture manifold is builtusing vectors of texture parameters 2110.

In embodiments, the vectors within a manifold are clustered 2110according to a clustering algorithm, such as, for example, K-MeansClustering (K-Means), as previously discussed. Those skilled in the artwill recognize that single or multiple clustering techniques arepossible for clustering algorithms, and that the choice of applicationof a particular clustering technique is not critical to the invention.

The cluster centers and respective cluster co-variances determined bythe clustering algorithm may provide the basis for manifold constraints2115. During model fitting, the likelihood of an invalid reconstructedobject image generated from the object model is reduced because theupdating of shape and texture parameters is constrained based on thedistance of these vectors to manifold cluster centers. In embodiments,an optimization fitting function may have a constraint based on theMahalanobis distance d_(M) of a parameter vector x to a cluster centeraccording to the equation:d _(M)(x)=(x−m)′C ⁻¹(x−m)   (25)

where m is the cluster center and C is the cluster covariance matrix.

In embodiments, the manifold constraints may be included 2020 as termsin the model, such as, for example, a shape constraint:K_(S)(p−m_(S))′C_(S) ⁻¹(p−m_(S))   (26)

where K_(S) is a shape constraint weight, p is a shape parameter, m_(S)is a cluster center in a shape manifold, and C_(S) is the shape manifoldcluster covariance matrix;

and a texture constraint:K_(a)(λ−m _(a))′C_(a) ⁻¹(λ−m_(a))   (27)

where K_(a) is a texture constraint weight, λ is a texture parameter,m_(a) is a cluster center in an texture manifold, and C_(a) is thetexture manifold cluster covariance matrix.

4. A Fourth Embodiment of a Method for Generating an Object Model

FIG. 22 depicts a method 2200 for generating an object model fittingsystem 345 comprising a balanced model and at least one manifoldconstraint according to various embodiments of the invention. Method2200 may be implemented in embodiments of object model builder 210comprising mesh generator 210, 3D shape reconstructor and modeler 320,subcomponent appearance modeler 330, model balancer 710, and manifoldbuilder 810.

a) Generating appearance and 3D Shape Models

In embodiments, a 3D shape model and an appearance model of an objectboth are generated 2205 based on a training data set of object images,and an object model comprising the appearance model constrained by the3D shape model is generated 2210, both in the same way as previouslydescribed for steps 1505 and 1510 in method 1500.

b) Model Balancing

In embodiments, steps 2215, 2220, 2225, and 2230 perform identificationand correction of training data set imbalances as described for steps1915, 1920, 1925, and 1930 in method 1900.

c) Generating Shape and Texture Manifolds

In embodiments, shape and texture manifolds are built 2240, and thesemanifold constraints are added to the object model fitting system 2245as described for steps 2015 and 2020 in method 2000.

D. Methods for Object Model Fitting

FIG. 23 depicts a method 2300 for fitting an object model to an inputobject image according to various embodiments of the invention. Method2300 may be implemented in embodiments of object model fitter 220.

FIG. 12 depicts an exemplary model fitting process according to variousembodiments of the invention. In embodiments, an input face image 1210is fit to a face model 1230 using a face model fitting system byestimating the values of parameters corresponding to 3D face pose, faceshape, and facial features of the input face image 1210 that minimizethe difference between the model 1230 and the input face image 1210. Inembodiments, the face model 1210 may be the appearance model in Equation(4) constrained by the global 3D shape model in Equation (2). Inembodiments, minimization is computed by iteratively updating parametervalues within a fitting function 1215 as well as associated weights,re-computing a face model estimate, and re-fitting the input face imageto the current face model estimate using, by way of illustration and notlimitation, the Levenberg-Marquardt (LM) algorithm, which is known tothose skilled in the art. Equation (5) is an exemplary fitting function1215 according to various embodiments of the invention.

Initially, the face model fitting system comprising the face model 3Dshape (p), pose (P), and texture (λ) parameters 1220 are set 2305 tomean values, although other initialization values may be used. Inembodiments, the input face image 1210 is warped to canonical view basedon the current 3D shape and pose estimate 1205. In embodiments, thecanonical view is the frontal view of the image. The current faceappearance model estimate 1230 is fitted 2310 to the warped input faceimage, and a fitting error is computed 2215. If a convergence conditionis met 2320, the current face appearance model estimate is set as thereconstructed face 2345. In embodiments, examples of convergenceconditions include the fitting error being less than a convergencethreshold value and the number of fitting iterations being greater thanan upper limit number of iterations. In various embodiments, more thanone convergence condition may be applied and convergence is determinedbased on at least one of the conditions being met.

If a convergence condition is not met 2320, estimates of the model 3Dshape (p), pose (P), and texture (λ) parameters 1220 are updated 2325based on the computed fitting error.

1. Robust Model Fitting

In embodiments, a face model fitting system may further compriseweighting values, and updating of parameters may also include updatingthe weighting values. The illustrated current face appearance modelestimate 1230 and warped input face image 1235 have been subdivided intosix facial subcomponents (left and right eyes, nose, mouth,forehead/brow line, and cheeks/chin area). In embodiments, robustfitting may be implemented by applying a weighting matrix 1225 tofeatures of pixels, regions of pixels, and/or facial subcomponentregions. Registration error weighting may enable robust fitting of theface model to an input face image containing occlusions (e.g., regionswith missing data) or other image artifacts (e.g., lens flare). Aspreviously discussed, computation of fitting errors may be used toidentify an occluded input face image because the regions correspondingto the occlusion may have relatively larger fitting errors. Inembodiments, those regions may be given smaller weights.

FIG. 14 illustrates an exemplary weighting of pixels based onregistration error within an occluded face image according to variousembodiments of the invention. The exemplary occluded face image 1410 hasbeen subdivided into triangular regions 1420 using DelaunayTriangulation, as previously described. In embodiments, the triangularregions (e.g., 1422 and 1424) within a face subcomponent correspondingto the occluded right eye 1412 of an occluded face image may have largerfitting errors, and thus those regions may be given smaller weights thanother regions (e.g., 1426) within the occluded face image. Since, inembodiments, weighting may be done at both a pixel level and asubcomponent level, the pixels in the occluded triangular regions havingthe greater fitting error (e.g., 1422) may be given lower weights thatthe pixels in other triangular regions (e.g., 1424) within the occludedsubcomponent. In the illustration, the regions within the occludedsubcomponent 1412 having the greater fitting error (e.g., 1422) arecolored darker than the other regions (e.g., 1424) within the occludedsubcomponent.

In embodiments, pose-based pixel weighting may be applied so that higherweights are given to pixels most directly facing the camera. FIG. 13illustrates an exemplary computation of pose-based weighting of pixelswithin subdivisions of a face image according to various embodiments ofthe invention. The exemplary face image 1305 has been subdivided intotriangular regions using Delaunay Triangulation, as previouslydescribed, and the camera axis 1315 (relative position of the camera1310) for each triangle is diagramed. In embodiments, the normal vector1325 for each triangle is computed, and the angle θ 1320 between thenormal vector and the camera axis is computed. In embodiments,pose-based model weights for the triangular regions may be based on adistribution of the computed angle θ of the normal vectors according toEquation (17), reproduced here:1−(min(θ,π/2)*2/π)⁴   (17)

Because the pose and shape parameters and mesh define a 3D shape meshcomprising triangles/subdivisions, each triangle in the 3D shape meshmay have a normal direction. The angles between the normals and thecamera axis in the 3D shape mesh can be used to update the weights.Thus, pose and shape parameters affect the weights because they are usedto determine the 3D shape, but, in embodiments, the weights are computedbased on the computed angle θ of the normal vectors, as described. Inembodiments, pose-based weights may be updated by determining an anglefor a triangle. All pixels in the mesh triangle may be associated withthe same angle or a normal angle may be interpolated at each pixel inthe triangle. A subcomponent of the object can be formed from aplurality of triangles and/or portions thereof, and the pose-basedweight associated with the subcomponent may be computed by averaging theangles associated with the pixels that form the subcomponent. Forexample, in embodiments, the normal angle for a subcomponent may becomputed by averaging the angles associated with pixels that form thesubcomponent.

2. Inverse Compositional Model Fitting Method

FIG. 24 depicts a method 2400 for an inverse compositional fitting of anobject model to an input object image according to various embodimentsof the invention. Method 2400 is an embodiment of method 2300 that isbased on an inverse compositional solution fitting function (e.g.,objective function 1215 in FIG. 12) that determines reconstructedtexture (λ) parameters based on solving for the minimized update to theshape parameters Δp in the image frame and then computing texturereconstruction A(λ) according to Equations (6) and (7), reproduced here:

$\begin{matrix}{\min\limits_{\lambda,{\Delta\; p}}{W{{{T\left( {{A(\lambda)};{\Delta\; p}} \right)} - {T\left( {I;p} \right)}}}^{2}}} & (6) \\{{A(\lambda)} = {A_{0} + {\sum\limits_{i = 1}^{N_{a}}{A\;\lambda}}}} & (7)\end{matrix}$

Initially, the model 3D shape (p), pose (P), and texture (λ) parameters1220 are set 2405 to mean values, although one skilled in the art shallrecognize that other values may be used to initialize the parameters.

The shape parameter update Δp in the mean shape frame may be calculated2410 according to Equation (8), reproduced here:Δp=R·E   (8)

where R is a shape parameter update matrix and E is a weightedregistration error vector E=(A₀−I) where A₀ is the mean appearance and Iis the current reconstructed image estimate.

In embodiments, R=−(SD′WSD)⁻¹SD′W and SD is a P_(S)×N steepest descentmatrix of shape eigenvectors (P_(S) is the number of shape modes and Nis the number of pixels) to a gradient of appearance means according tothe equation:

$\begin{matrix}{{SD} = \left( {{\nabla A_{0}}\frac{\partial W}{\partial p}} \right)} & (19)\end{matrix}$

Thus, Δp may be also be written as Δp=−(SD′WSD)⁻¹SD′W(A₀−I). It shouldbe noted that if W is constant, then R does not change and may bepre-computed. In embodiments, as discussed herein, different weightingscan be applied via the weighting matrix, W.

A new shape estimate p is computed 2415 by merging Δp into the shapeparameters in the image frame (1235 in FIG. 12) according to Equations(9), (10), (11), and (12), reproduced here:

$\begin{matrix}{S_{shape} = {{T\left( {S_{0};p} \right)} + {T\left( {S_{0};{{- \Delta}\; p}} \right)}}} & (9) \\{\min\limits_{\Delta\; p}{{S_{shape} - \left( {S_{o} + {S\left( {p - {\Delta\;\hat{p}}} \right)}} \right)}}^{2}} & (10) \\{{\Delta\;\hat{p}} = {{- (I)^{- 1}}S^{\prime}{W\left( {S_{shape} - \left( {S_{0} + {Sp}} \right)} \right)}}} & (11) \\{p = {p_{0} - {\Delta\;\hat{p}}}} & (12)\end{matrix}$

That is, S_(shape) is determined, where S_(shape) is the mean shape (S₀)warped by the shape parameter update, −Δp, and then warped to the imageframe by the current shape parameters, p, T(S₀;p) is the current shapeestimate, T(S₀;−Δp) is the delta shape, and where S is the set of shapeeigenvectors, S′ is the transpose of S, S_(p) is the modified set ofshape eigenvectors, I is the set of image texture eigenvectors, and W isa weighting matrix. Note that T(x,y) defines a warping of the shape, x,by the parameters, y. The shape update, Δ{circumflex over (p)}, in theimage frame may be determined according to the Equations (10) and (11).It should be noted that in this embodiment no weighting matrix isimplemented here because for the shape vectors. However, one skilled inthe art shall recognize that the shape model could be weighted. Theupdated shape parameters p in the image frame may be computed using theprevious best shape parameters, and Δ{circumflex over (p)}, according toEquation (12). The best shape parameters are the parameters from theprevious iteration that resulted in the lowest registration error.

The registration error vector E is calculated 2420 based on the imagetexture λ that has been extracted from the image frame which has beenupdated based on p. The image texture λ may be computed according toEquations (14) and (15), reproduced here:

$\begin{matrix}{\min\limits_{\lambda}{W{{A_{o} + {A\;\lambda} - {T\left( {I;p} \right)}}}^{2}}} & (14) \\{\lambda = {{- \left( {A^{\prime}{WA}} \right)^{- 1}}A^{\prime}{W\left( {A_{o} - {T\left( {I;p} \right)}} \right)}}} & (15)\end{matrix}$

and then E may be computed according to Equation (16), reproduced here:

$\begin{matrix}{E = {A_{0} + {\sum\limits_{i = 1}^{N_{Ai}}{A\;\lambda}} - I}} & (16)\end{matrix}$

In embodiments, the value of Δ{circumflex over (p)} may not haveproduced a decrease in the registration error vector E. If E did notdecrease 2425, the value of Δ{circumflex over (p)} may be reduced 2435by dividing it by a division factor, such as, for example, a factor of2:

$\begin{matrix}{{\Delta\;\hat{p}} = \frac{\Delta\;\hat{p}}{2}} & (28)\end{matrix}$

and then computing a new registration error vector E (steps 2415 and2420) using the reduced Δ{circumflex over (p)}. If E did not decrease2425, steps 2435, 2415, and 2420 may be repeated until a predeterminedmaximum number of Δ{circumflex over (p)} reductions has been made 2430.

If a decrease in the registration error vector E occurs 2425, the shapeparameter estimate p is set. If the decrease in E is greater than athreshold or a predetermined maximum number of iterations has not beenreached 2445, a new shape parameter estimate p is iteratively computed(steps 2410 through 2440).

If the decrease in E is less than the threshold or the predeterminedmaximum number of iterations has been reached 2445, or if the maximumnumber of reductions of the value of Δ{circumflex over (p)} have beenmade 2430, the value of E is compared to a threshold value 2450. If E isless than a threshold 2455, model fitting has converged. If E is greaterthan the threshold 2460, model fitting has failed to converge.

3. Object Model Fitting With Manifold Constraints

FIG. 25 depicts a method 2500 for applying a manifold constraintaccording to various embodiments of the invention. Method 2500 may beimplemented in embodiments of step 2325 of method 2300.

In embodiments, identification 2505 of the nearest manifold cluster to aparameter vector x may be based on the Mahalanobis distance of theparameter vector x to a cluster center according to Equation (25), aspreviously described. In embodiments, manifold constraints may beapplied 2510 to the update of the parameter vector x based on thedistance to the nearest manifold cluster center as described inEquations (26) and (27).

In embodiments, shape and appearance manifold constraints may be appliedalong with a fitting function (such as, for example, the fittingfunction exemplified in Equation (5)), such as in the equation:

$\begin{matrix}{{\min\limits_{\lambda,p,P}{W\left\lbrack {{\sum\limits_{i = 1}^{N_{D}}\left( {A_{i\; 0} + {\sum\limits_{j = 1}^{N_{Ai}}{\lambda_{ij}A_{ij}}}} \right)} - {T\left( {{I;p},P} \right)}} \right\rbrack}_{2}^{2}} + {{K_{s}\left( {p - m_{s}} \right)}^{\prime}{C_{s}^{- 1}\left( {p - m_{s}} \right)}} + {{K_{a}\left( {\lambda - m_{a}} \right)}^{\prime}{C_{a}^{- 1}\left( {\lambda - m_{a}} \right)}}} & (29)\end{matrix}$

where K_(S) is a shape constraint weight, m_(S) is a cluster center in ashape manifold, and C_(S) is the shape manifold cluster covariancematrix., and where K_(a) is a texture constraint weight, m_(a) is acluster center in an texture manifold, and C_(a) is the texture manifoldcluster covariance matrix.

In embodiments of method 2400, the solution for a shape parameter updateΔp (See Equations (10) and (11)) may include shape manifold constraintssuch as in the equations:

$\begin{matrix}{{\min\limits_{\Delta\; p}{{S_{shape} - \left( {S_{o} + {S\left( {p - {\Delta\;\hat{p}}} \right)}} \right)}}^{2}} + {K_{s}\left\lbrack {\left( {p + {\Delta\;\hat{p}} - m_{s_{s}}} \right)^{\prime}{C_{s_{s}}^{- 1}\left( {p + {\Delta\;\hat{p}} - m_{s_{s}}} \right)}} \right\rbrack}} & (30) \\{{\Delta\;\hat{p}} = {{{- \left( {I + {K_{s}C_{s_{s}}^{- 1}}} \right)^{- 1}}S^{\prime}{W\left( {S_{shape} - \left( {S_{0} + {Sp}} \right)} \right)}} - {{K_{s}\left( {I + {K_{s}C_{s_{s}}^{- 1}}} \right)}^{- 1}{C_{s}^{- 1}\left( {p - m_{s}} \right)}}}} & (31)\end{matrix}$

In embodiments of method 2400 in which shape reconstruction is weighted,manifold constraints also may be applied, such as in the equation:Δ{circumflex over (p)}=−(S′WS+K _(s) C _(S) _(s) ¹)⁻¹ S′W(S _(shape)−(S₀ +Sp))−K _(s)(S′WS+K _(s) C _(S) ⁻¹)⁻¹ C _(S) _(s) ⁻¹(p−m _(s))   (32)

In embodiments of method 2400, the solution for an extracted textureparameter λ (See Equations (14) and (15)) may include texture manifoldconstraints such as in the equations:

$\begin{matrix}{{\min\limits_{\lambda}{W{{A_{o} + {A\;\lambda} - {T\left( {I;p} \right)}}}^{2}}} + {K_{a}\left\lbrack {\left( {\lambda - m_{a}} \right)^{\prime}{C_{a_{t}}^{- 1}\left( {\lambda - m_{a_{t}}} \right)}} \right\rbrack}} & (32) \\{\lambda = {{{- \left( {{A^{\prime}{WA}} + {K_{a}C_{a_{t}}^{- 1}}} \right)^{- 1}}A^{\prime}{W\left( {A_{o} - {T\left( {I;p} \right)}} \right)}} + {{K_{a}\left( {{A^{\prime}{WA}} + {K_{a}C_{a}^{- 1}}} \right)}^{- 1}C_{a}^{- 1}m_{a}}}} & (33)\end{matrix}$

E. Computing System Implementations

It shall be noted that the present invention may be implemented in anyinstruction-execution/computing device or system capable of processingthe image data, including without limitation, a general-purpose computerand a specific computer, such as one intended for graphics processing.The present invention may also be implemented into other computingdevices and systems, including without limitation, a digital camera, aprinter, a scanner, a multiple function printer/scanner, a facsimilemachine, a multimedia device, and any other device that processes,captures, transmits, or stores an image. Furthermore, within any of thedevices, aspects of the present invention may be implemented in a widevariety of ways including software, hardware, firmware, or combinationsthereof. For example, the functions to practice various aspects of thepresent invention may be performed by components that are implemented ina wide variety of ways including discrete logic components, one or moreapplication specific integrated circuits (ASICs), and/orprogram-controlled processors. It shall be noted that the manner inwhich these items are implemented is not critical to the presentinvention.

FIG. 26 depicts a functional block diagram of an embodiment of aninstruction-execution/computing device 2600 that may implement or embodyembodiments of the present invention. As illustrated in FIG. 26, aprocessor 2602 executes software instructions and interacts with othersystem components. In an embodiment, processor 2602 may be a generalpurpose processor such as (by way of example and not limitation) an AMDprocessor, an INTEL processor, a SUN MICROSYSTEMS processor, or aPOWERPC compatible-CPU, or the processor may be an application specificprocessor or processors. A storage device 2604, coupled to processor2602, provides long-term storage of data and software programs. Storagedevice 2604 may be a hard disk drive and/or another device capable ofstoring data, such as a computer-readable media (e.g., diskettes, tapes,compact disk, DVD, and the like) drive or a solid-state memory device.Storage device 2604 may hold programs, instructions, and/or data for usewith processor 2602. In an embodiment, programs or instructions storedon or loaded from storage device 2604 may be loaded into memory 2606 andexecuted by processor 2602. In an embodiment, storage device 2604 holdsprograms or instructions for implementing an operating system onprocessor 2602. In one embodiment, possible operating systems include,but are not limited to, UNIX, AIX, LINUX, Microsoft Windows, and theApple MAC OS. In embodiments, the operating system executes on, andcontrols the operation of, the computing system 2600.

An addressable memory 2606, coupled to processor 2602, may be used tostore data and software instructions to be executed by processor 2602.Memory 2606 may be, for example, firmware, read only memory (ROM), flashmemory, non-volatile random access memory (NVRAM), random access memory(RAM), or any combination thereof. In one embodiment, memory 2606 storesa number of software objects, otherwise known as services, utilities,components, or modules. One skilled in the art will also recognize thatstorage 2604 and memory 2606 may be the same items and function in bothcapacities. In an embodiment, one or more of the components of FIGS. 2,3, 7, 8, 10, and 11 may be modules stored in memory 2604, 2606 andexecuted by processor 2602.

In an embodiment, computing system 2600 provides the ability tocommunicate with other devices, other networks, or both. Computingsystem 2600 may include one or more network interfaces or adapters 2612,2614 to communicatively couple computing system 2600 to other networksand devices. For example, computing system 2600 may include a networkinterface 2612, a communications port 2614, or both, each of which arecommunicatively coupled to processor 2602, and which may be used tocouple computing system 2600 to other computer systems, networks, anddevices.

In an embodiment, computing system 2600 may include one or more outputdevices 2608, coupled to processor 2602, to facilitate displayinggraphics and text. Output devices 2608 may include, but are not limitedto, a display, LCD screen, CRT monitor, printer, touch screen, or otherdevice for displaying information. Computing system 2600 may alsoinclude a graphics adapter (not shown) to assist in displayinginformation or images on output device 2608.

One or more input devices 2610, coupled to processor 2602, may be usedto facilitate user input. Input device 2610 may include, but are notlimited to, a pointing device, such as a mouse, trackball, or touchpad,and may also include a keyboard or keypad to input data or instructionsinto computing system 2600.

In an embodiment, computing system 2600 may receive input, whetherthrough communications port 2614, network interface 2612, stored data inmemory 2604/2606, or through an input device 2610, from a scanner,copier, facsimile machine, or other computing device.

One skilled in the art will recognize no computing system is critical tothe practice of the present invention. One skilled in the art will alsorecognize that a number of the elements described above may bephysically and/or functionally separated into sub-modules or combinedtogether.

It shall be noted that embodiments of the present invention may furtherrelate to computer products with a computer-readable medium that havecomputer code thereon for performing various computer-implementedoperations. The media and computer code may be those specially designedand constructed for the purposes of the present invention, or they maybe of the kind known or available to those having skill in the relevantarts. Examples of computer-readable media include, but are not limitedto: magnetic media such as hard disks, floppy disks, and magnetic tape;optical media such as CD-ROMs and holographic devices; magneto-opticalmedia; and hardware devices that are specially configured to store or tostore and execute program code, such as application specific integratedcircuits (ASICs), programmable logic devices (PLDs), flash memorydevices, and ROM and RAM devices. Examples of computer code includemachine code, such as produced by a compiler, and files containinghigher level code that are executed by a computer using an interpreter.

While the invention is susceptible to various modifications andalternative forms, specific examples thereof have been shown in thedrawings and are herein described in detail. It should be understood,however, that the invention is not to be limited to the particular formsdisclosed, but to the contrary, the invention is to cover allmodifications, equivalents, and alternatives falling within the scope ofthe appended claims.

1. A computer-readable medium carrying one or more sequences ofinstructions, wherein execution of the one or more sequences ofinstructions by one or more processors causes the one or more processorsto form an object model fitting system based on a set of training imagesby performing the steps comprising: generating a three-dimensional (3D)object shape model comprising shape parameters, the 3D object shapemodel based on the set of training images; generating an objectappearance model comprising texture parameters, the object appearancemodel based on the set of training images; forming an interim objectmodel comprising the object appearance model constrained by the 3Dobject shape model; using the interim object model to generate at leastone set of manifold constraints associated with a manifold, the manifoldbased on a vector of a type of model parameters, the type of modelparameters comprising shape and texture; and forming the object modelfitting system comprising the interim object model and the at least oneset of manifold constraints.
 2. The computer-readable medium of claim 1wherein the object appearance model comprises a set of objectsubcomponent appearance models.
 3. The computer-readable medium of claim2 wherein the object model is a face model and the training images areface images.
 4. The computer-readable medium of claim 3 wherein usingthe interim object model to generate a least one set of manifoldconstraints associated with a manifold comprises: obtaining a set ofmodel parameter vector instances by: for each image from a set of imagesselected from the set of training images, fitting an image to theinterim object model to obtain a reconstructed object image instance;extracting a model parameter vector instance from the reconstructedobject image instance; and adding the model parameter vector instance tothe set of model parameter vector instances; forming the manifold usingthe set of model parameter vector instances; and generating a set ofmanifold constraints by applying a cluster analysis to the set of modelparameter vector instances.
 5. The computer-readable medium of claim 4wherein generating the set of manifold constraints is based on a set ofcluster centers and a set of cluster co-variances associated with theset of cluster centers.
 6. The computer-readable medium of claim 5wherein the cluster analysis is K-Means clustering.
 7. Acomputer-readable medium carrying one or more sequences of instructions,wherein execution of the one or more sequences of instructions by one ormore processors causes the one or more processors to fit an input imageto an object model comprising at least one set of manifold constraintsand a three-dimensional (3D) object active appearance model (AAM)comprising a 3D object shape model and an object appearance modelcomprising a set of object subcomponent appearance models, by performingthe steps comprising: [a] forming a first image by projecting a 3Dobject shape and pose estimate onto the input image using the 3D objectshape model; [b] forming a second image using an object appearance modelestimate obtained using the object appearance model; [c] calculating aset of model fitting errors using the first image and the second image;and [d] responsive to a stop condition not being satisfied, forming anupdated 3D object shape and pose estimate and an updated objectappearance model estimate by updating model parameter values using atleast some of the set of model fitting errors and the at least one setof manifold constraints, and iterating by returning to step [a].
 8. Thecomputer-readable medium of claim 7 wherein the at least one set ofmanifold constraints comprises shape parameters associated with the 3Dobject shape model.
 9. The computer-readable medium of claim 7 whereinthe at least one set of manifold constraints comprises textureparameters associated with the object appearance model of the 3D AAM.10. The computer-readable medium of claim 7 wherein the at least one setof manifold constraints are generated using a texture manifold formedusing texture parameters associated with the object appearance model andusing a shape manifold formed using shape parameters associated with the3D object shape model.
 11. The computer-readable medium of claim 7wherein the 3D object AAM is a 3D face AAM and the input image is a faceimage.
 12. The computer-readable medium of claim 11 wherein a manifoldconstraint is associated with a distance of a model parameter vectorinstance to a cluster center within a manifold, the manifold comprisinga set of model parameter vector instances obtained by fitting a trainingset of images to an interim object AAM comprising the 3D object shapemodel and the object appearance model.
 13. The computer-readable mediumof claim 12 wherein a cluster analysis is applied to the set of modelparameter vector instances to obtain a set of cluster centers and a setof cluster co-variances associated with the set of cluster centers, andthe cluster center is from the set of cluster centers.
 14. Thecomputer-readable medium of claim 13 wherein a distance of a modelparameter vector instance to the cluster center within the manifold is aMahalanobis distance.
 15. A system for fitting an input image comprisingpixels to an object model, the system comprising: an object modelfitter, coupled to receive the input image, that fits the input imageusing an object model fitting system comprising at least one set ofmanifold constraints and a three-dimensional (3D) object activeappearance model (AAM) comprising a 3D object shape model and an objectappearance model comprising a set of object subcomponent appearancemodels.
 16. The system of claim 15 wherein the object model fitter fitsthe input image using an object model fitting system by performing thesteps comprising: [a] forming a first image by projecting a 3D objectshape and pose estimate onto the input image using the 3D object shapemodel; [b] forming a second image using an object appearance modelestimate obtained using the object appearance model; [c] calculating aset of model fitting errors using the first image and the second image;and [d] responsive to a stop condition not being satisfied, forming anupdated 3D object shape and pose estimate and an updated objectappearance model estimate by updating model parameter values using atleast some of the set of model fitting errors and the at least one setof manifold constraints, and iterating by returning to step [a].
 17. Thesystem of claim 15 further comprising an object model builder, coupledto receive a training set of images, that forms the 3D object modelcomprising the 3D object shape model, the object appearance model, andthe at least one set of manifold constraints.
 18. The system of claim 17wherein the object model builder comprises: a 3D shape reconstructor andmodeler, coupled to receive shape information for the set of trainingimages, the 3D shape reconstructor and modeler forming the 3D objectshape model based on the shape information; and a subcomponentappearance modeler, coupled to receive the set of training images, thesubcomponent appearance modeler forming the set of object subcomponentappearance models using the set of training images; and a manifoldbuilder, coupled to receive the set of training images and an interim 3Dobject model comprising the set of object subcomponent appearance modelsand the 3D shape model, that generates at least one manifold that addsat least one constraint to the 3D object model, the at least onemanifold based on a vector of a type of model parameters, the type ofmodel parameters comprising shape and texture.
 19. The system of claim18 wherein generating at least one manifold that adds at least oneconstraint is performed by the steps comprising: obtaining a set ofmodel parameter vector instances by: for each image from a set of imagesselected from the set of training images, fitting an image to theinterim 3D object model to obtain a reconstructed object image;extracting a model parameter vector instance from the reconstructedobject image; and adding the model parameter vector instance to the setof model parameter vector instances; forming the manifold using the setof model parameter vector instances; and generating a set of manifoldconstraints by applying a cluster analysis to the set of model parametervector instances.
 20. The system of claim 19 wherein generating the setof manifold constraints is based on a set of cluster centers and a setof cluster co-variances associated with the set of cluster centers.